Any way to control light halos in the compositor based on emission level?
With the glare node you can control the threshold at which the effect kicks in.

In this example a the cubes have different emission values. Form left to right they double in intensity. 

Using the threshold control on the Glare node you can determine at what point the effect starts:







How would you know which values to use? You can see the values for the rendered image if you left click and drag the mouse over the image. You'll see some values at the bottom of the screen On the left side you'll see values for RGB (red, green and blue). 





Those values are the scene referred values before any color transformation. Konwing those values you should be able to determine the value for the glare threshold.

Update for 2.8:

To show the sampler in the Image Editor window press the right mouse button. The sampler window will appear at the bottom of the screen.